1. Field of the Invention
The invention relates to digital signal processing and, more particularly, to a method for filtering a chromatogram.
2. Description of the Related Art
In chromatography, a sample of a substance mixture to be analyzed is passed through a chromatographic separating device. Because of different migration rates through the separating device, the analytes, i.e., the individual substances of the substance mixture, reach the output of the separating device at different times and are successively detected at this point by a suitable detector. The time that the analytes require to migrate through the separating device is referred to as the retention time. As its measurement signal, the detector generates a chromatogram that consists of a baseline and a number of peaks corresponding to the separated substances. In practice, the chromatogram is affected by noise, with the individual peaks standing out more or less clearly from the signal noise. The detection limit of an analyte is defined as a predetermined multiple of the noise. That is, the peak height measured from the noise-free baseline, i.e., from the average value of the noise, must be at least the predetermined multiple of the noise.
With well-resolved peaks, the peak area above the noise-free baseline is proportional to the concentration of the analyte. The peak area, in contrast to the peak height, provides accurate measurement results even for nonsymmetrical peaks.
In order to isolate the analytical information, i.e., the peaks, the chromatogram is smoothed by lowpass filtering. Smoothing algorithms suitable for this purpose are, for example, a moving average or the Savitzky-Golay filter. The lower the limit frequency of the lowpass filter or the greater the filter length of the Finite Impulse Response (FIR) filter used, the better the smoothing that can be obtained. With increasing smoothing, however, the peaks may also be deformed so that the measurement accuracy is reduced. Depending on which substance mixtures are to be analyzed, the measurement applications, for example, different separating columns with different interconnection, and measurement conditions within an application, for example, different temperature and pressure profiles in the separating device, may be very different and lead to correspondingly different chromatograms, which necessitates differently dimensioned filters for smoothing the chromatograms.